Null voltage indicating circuit



May 6, 1952 H. BEDFORD ET AL. 2,595,754

NULL VOLTAGE INDICATING CIRCUIT Original Filed Nov. 30, 1945 a l5D.C.VOLTAGE /7 I Q 5 .1!

URED-p 1, J N fi LEn L E T 4 DC. VOLTAGE J O BE MEASURED DR NULLEDA.C.VDLTAGE O BE MEASURED OR NULLED INVENTORfi Leslie HQIZ'JBI'i'BedfordJohn Bell Ez-ic Mibes Langiimuc attorney Patented May 6, 1952 NULLVOLTAGE INDICATING CIRCUIT Leslie Herbert Bedford, London, and John Belland Eric Miles Langh assignors to A. C. land, a British company ember30, 1943, Serial No. Divided and this application July 26,

Original application Nov am, Teddington, England, ssor Limited, London,Eng- 1946, Serial No. 686,478. In Great Britain November 30, 1942 4Claims.

This invention relates to thermionic valve' circuits for voltagemeasurement or indication.

According to the invention there is provided a null voltage indicatingcircuit, comprising a pair of similar thermionic valves having similaranode loads, a direct current null indicating meter connected betweenthe anodes of said valves, a common cathode load for said valvesconsisting of two resistance potentiometers, connected in parallel witheach other and with a smoothing condenser, means to excite the anodecircuit with alternating voltage, means to connect the grid of one ofsaid valves to a tapping on one of said potentiometers and means toinject the voltage to be nulled between the grid of the other of saidvalves and a tapping on the other of said potentiometers so selectedthat when said voltage is zero a null indication is obtained on saidmeter.

The circuit according to the invention is capable of use for nullingeither a direct voltage, or an alternating voltage of the same frequencyas the alternating voltage which excites the anode circuit; but in thelatter case the voltage to be nulled must not be in quadrature with thatwhich excites the anode circuit.

In a preferred form of the invention, the circuit is arranged formeasuring a direct voltage injected in place of said voltage to benulled, the tapping to which said voltage is connected being mademoveable so as to add, in series with said injected direct voltage, anopposing direct voltage being a fraction of the total voltage developedacross said potentiometers, and the degree of said movement necessary toproduce a null indication being taken as the measure of the directvoltage injected.

,In another preferred form of the invention, the circuit is arranged foralternatively measuring a direct voltage which is relatively invariable,and measuring or nulling at least one other voltage, said circuit havinganother resistance potentiometer connected in parallel into said cathodeload, and having switching means whereby the grid of one of said valvesmay be connected alternately 1) through the source of said relativelyinvariable direct voltage to the moveable tapping of a potentiometerreserved for measuring that voltage, and (2) through the source ofanother voltage to be measured or nulled to the tapping of anotherpotentiometer.

A diagram of a thermionic valve circuit embodying the invention is shownin the accompanying drawing. This circuit; may be employed either formeasuring a direct voltage, or for observing when a direct voltage or analternating voltage is brought to null value. It is immaterial whethereither side of the applied alternating or direct voltage is earthed.

A pair of similar valves I, 2 have their anode circuits connected acrossan unearthed alterhating voltage supply derived through a transformer l2from source 16, which may, for example, be a 50 volt 50 cycle source.These anode circuits comprise separate and similar anode load resistors3, 4, and a common cathode load consisting of a plurality of resistancepotentiometers 5, 6, I, 8, all in parallel, provided with a by-passsmoothing condenser [3. The control rid of valve 2 is connected to themoveable tapping on potentiometer 5. The control grid of valve 1 isarranged to be connected through switch 9 alternatively, either directlyto the centre tap on potentiometer 6, or through a pair of inputterminals IE to the centre tap on potentiometer 6, or through a pair ofinput terminals It to the moveable tapping on the potentiometer 8, orthrough a pair of input terminals M to the moveable tapping onpotentiometer l. A centre-zero direct current indicator II is connecteddirectly between the anodes of the valves l, 2. No point of the valvecircuit is earthed.

In order to adjust the circuit for any inequalities, the switch 9 isfirst placed in the attitude to connect the control grid of valve ldirectly to the centre tap on potentiometer 6, and the moveable tappingon potentiometer 5 is then moved until a null indication is obtained onindicator l I.

Switch 9 may then be thrown to the attitude in which the control grid ofvalve I is connected through terminals It to the moveable tapping onpotentiometer 8. A direct voltage to be measured is applied betweenterminals [6. If the value of this direct voltage input is zero, a nullindication will be obtained when the tapping on potentiometer 8 is movedto its middle position. Any departure from the middle position which maybe necessary in order to obtain a null indication is a measure of thedirect voltage input at terminals l9. This voltage is equal to thevoltage drop in potentiometer ii between the mid position and theposition giving a null reading.

If it is desired to read the value of the direct voltage input atterminals Hi, the moveable tapping on potentiometer 8 may be providedwith a graduated scale. If, however, it is merely required that anotherinstrument shall be set in accordance with that voltage input, then amechanical coupling may be employed between the tapping on potentiometerinstrument.

8 and the said other If switch 9 is now thrown to the attitude in whichthe control grid of valve I is connected through terminals I4 to themoveable tapping on potentiometer I, then this moveable tapping can beadjusted to obtain a null indication without disturbing the setting ofthe moveable tapping of potentiometer 8. Just as the setting ofpotentiometer 8 for a null indication was a measure of the directvoltage input between terminals I0,

so the setting of the moveable tapping of- I which yields a nullindication is a measure of a direct voltage input applied betweenterminals I4.

Any desired number of further potentiometers similar to I, 8 may beprovided, each having a further pair of input terminals connectedbetween its moveable tapping and a further position of switch 9, for themeasurement of further direct input voltages.

The provision of a separate potentiometer for measuring each of aplurality of relatively invariable direct voltages gives the advantagethat the settings of the moveable tappings need not be disturbed whenchanging over from one to another. Thus the settings of the moveabletappings continue to represent the last readings of each, and willusually be approximately correct when returns are made to each forcorrection of their settings.

If switch 9 is thrown to the attitude in which the control grid of valveI is connected through terminals I to the centre tap on potentiometer 6,the null reading of indicator II will be disturbed if any voltage,direct or alternating, is applied between terminals I5. If, for example,it is desired to measure an alternating voltage in phase with that ofsource I6, this may be applied, in series with the output of a linearpotentiometer I1 connected across the source I6, to the primary windingof a transformer I8. The secondary winding of the transformer I8 will beconnected between terminals I5. The potentiometer M will then beadjusted until a null reading is obtained on indicator I I. Then theapplied alternating voltage must be equal in amplitude to the outputvoltage of the linear potentiometer I1, and its value is indicated bythe setting of the potentiometer.

It is an important feature of the invention that a reversal of phase ofan alternating voltage applied between terminals I5 produces a reversalof the current through indicator II, so a sense indication is obtainedwhen nulling an alternating voltage.

This application is a division of application Serial No. 512,402, filedNovember 30, 1943.

We claim:

1. A null voltage indicating circuit, comprising a pair of similarthermionic valves each containing at least an anode, a cathode and agrid and having similar anode loads, a direct current null indicatingmeter connected between the anodes of said valves, an alternatingvoltage source for exciting the anode circuit of said valves, a commoncathode load for said valves connected in series with the cathodes andsaid alternating voltage source and comprising two resistancepotentiometers connected in parallel with each other and a smoothingcondenser in parallel with said potentiometers, means to connect thegrid of one of said valves directly to a tapping on one of saidpotentiometers and means to inject the voltage to be nulled in serieswith the grid of the other of said valves and a tapping on the second ofsaid potentiometers, said tapping on the sec- 0nd of said potentiometersbeing so selected that when said voltage is zero a null indication isobtained on said meter, said circuit being so constructed and arrangedthat when an alternating voltage is injected for nulling, a reversal ofphase of said voltage produces a reversal of the current flow throughsaid meter so as to give a sense indication.

2. A circuit according to claim 1, arranged for measuring a directvoltage injected in the place of said voltage to be nulled, wherein thetapping on said second potentiometer to which said voltage is connectedis made moveable so as to add in series with said injected directvoltage, an opposing direct voltage being a fraction of the totalvoltage developed across said potentiometers, and wherein the degree ofsaid movement necessary to produce a null indication is a measure of thedirect voltage injected.

3. A circuit according to claim 2, for alternatively measuring a directvoltage which is relatively invariable, and measuring or nulling atleast one other voltage, said circuit having a third resistancepotentiometer connected in parallel into said cathode load, and. havingswitching means for connecting the grid of one of said valvesalternately through the source of said relatively invariable directvoltage to the moveable tapping of the second potentiometer and throughthe source of said other voltage to be measured or nulled to the tappingof said third potentiometer.

4. A null voltage indicating circuit comprising a pair of similarthermionic valves each containing at least an anode, a cathode and agrid, a direct current null indicating meter connected between theanodes of said valves, a circuit including an alternating voltagesource, similar fixed resistances connecting the anodes of said valvesto said source and a common cathode load comprising a plurality ofresistance potentiometers in parallel with each other and a smoothingcondenser in parallel with said potentiometers, said load beingconnected in series with the cathodes of said valves and saidalternating voltage source, means for connecting the grid of one of saidvalves directly to a tapping on one of said potentiometers, and switchmeans for injecting a voltage to be nulled in series with the grid ofthe other of said valves and a tapping on at least one other of saidpotentiometers, said last named tapping being so selected that when saidvoltage is zero a null indication is obtained on said meter.

LESLIE HERBERT BEDFORD. J OHN BELL. ERIC MILES LANGHAM.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,919,538 Stone July 25, 19332,143,219 Wenger Jan. 10, 1939 2,200,055 Burnett May 7, 1940 2,207,976Ferrell July 16, 1940 2,226,255 Percival Dec. 24, 1940 2,264,197Hadfield Nov. 25, 1941 2,305,307 Wellenstein Dec. 15, 1942 2,329,073Mitchell et al Sept. 7, 194? FOREIGN PATENTS Number Country Date 526,869Great Britain Sept. 26, 1940

